P800KO-422 Closet Flange with knockout. Nested fitting. Provides a 3" slip socket (on the bottom) by closet flange on the top. However, the bottom will also glue directly into 4" pipe. for 3" going to 4". Use this part if you want to seal the end of a 4 pipe with the flange. (most flanges are open). It won't hold a lot of pressure (no potato guns), but it'll hold a column of water just fine.